DATING PRE-ILLINOIAN GLACIATIONS EAST OF THE MISSISSIPPI RIVER

The area between the Mississippi and Illinois Rivers is generally recognized as the approximate boundary between lobes of the western and eastern Laurentide Ice Sheet (LIS). Western source pre-Illinoian tills are widely present beyond the limits of younger glaciations throughout much of Iowa, Nebraska and northern Missouri, and these areas have well-established till stratigraphies with at least some age control. The most detailed chronology of pre-Illinoian glaciation is for northeast Missouri, based on cosmogenic-nuclide burial dating. There, six dated tills within five lithostratigraphic units range in age from ~2.4 Ma to ≤ 0.4 Ma. However, the oldest of these and another undated till in extreme eastern Missouri were likely deposited by eastern lobes of the LIS, based on various indicators of provenance, so deposits from these glaciations should also be present farther to the east.

The chronology of pre-Illinoian tills east of the Mississippi is not as well developed, but burial dating should be applicable there as well. The basis of this method is the ratio of ²⁶Al:¹⁰Be within quartz grains produced by cosmogenic radiation at or near the ground surface for at least several tens of thousands of years, i.e. within mature paleosols. These nuclides are produced in the ratio 6.75:1, but if a paleosol is buried deeply enough by a younger till while preserving part of the solum, nuclide production (nearly) stops and the ratio decreases in proportion to burial time due to differential decay. Thus, the measured nuclide ratio gives the age of the overlying till. Under best conditions this method can provide ages to within approximately 0.05 myr at a given site for Pleistocene deposits older than ~0.5 Ma. Deposits as young as ~0.2 Ma can also be dated, but with increasing error limits. Future applications of burial dating in the eastern U.S. could address the timing of major ice growth and advances of the eastern LIS, along with the degree of synchronicity with its western sector, as well as the areal extent of specific Early – Middle Pleistocene glaciations.